This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Murine PyV is a double stranded, none enveloped virus. Its capsid (50 nm in diameter) is composed of 72 pentamers of the major coat protein VP1 and of VP2 and VP3 proteins buried within the capsid. After entry and migration to the cell host nucleus, the mPyV gene expression can be divided into two phases (3, 4). The early phase sees nonstructural viral proteins such as small, middle and large T antigen to be synthesized. Those proteins highly interact with the host cell key proteins. As a result, the virus takes control of cellular mechanisms like;cell cycle progression towards S-phase as well as preventing the cell to engage apoptosis process. The early phase leads to viral DNA replication exploiting the host nuclear machinery. During the later phase, the structural proteins (VP1, VP2, VP3) are expressed and de novo viral particles are assembled in the nucleus. The goal of the collaboration is to combine immunocytochemistry data on EM prepared infected 3T3 cells coming from the Garcea lab and ultrastructural information collected in the facility to understand better the way Polyoma virus particles assembled in the cell nucleus as well as how PML bodies are involved in this process.